There is known an antenna array apparatus arranging a plurality of antennas to thereby control the directivity thereof, as an antenna apparatus included in a transmitter of a radio communications system.
By using such an array antenna apparatus, a beam having an acute directivity can be formed in a desired direction. This enables control, to raise the frequency utilization efficiency by reducing the repeated distance at the same frequency, or to control the null point in order not to radiate a radio wave in unwanted directions.
The array antenna, generally, has a plurality of antennas. The antennas are respectively connected with power amplifiers for supplying signals. RF signals generated are amplified by the power amplifiers and then radiated through the antennas. However, the nonlinear distortion caused upon amplification by the power amplifier forms a factor to deteriorate beam control accuracy over the array antenna apparatus. For this reason, there is proposed, as a countermeasure, an array antenna apparatus having distortion compensator circuits arranged for all or part of the power amplifiers connected one-to-one to the antennas.
On the array antenna apparatus, provided are distortion compensator circuits on part or all of the antenna arrays. The IQ signal is added by such a distortion as to compensate for a nonlinear distortion occurred in the power amplifier. Due to this, the array antenna apparatus is configured high in beam control accuracy, small in size but low in consumption power.
FIG. 13 shows an array antenna apparatus having distortion compensators only for the power amplifiers of part of antenna arrays.
In FIG. 13, a signal generating section 90 is to output therefrom a transmission IQ signal 902.
A beam-direction control section 913 is to output therefrom a beam-direction control signal 914.
An amplitude-phase control section 903 is to input therein a transmission IQ signal 902 and beam-direction control signal 914 and to output a transmission IQ signal 904 controlled in amplitude and phase.
A frequency converting section 905 is to input therein a transmission IQ signal 904 controlled in amplitude and phase and to output an RF signal 906.
A power amplifier 907 is to input therein an RF signal and to output an amplified RF signal 909.
An antenna 909 is to input therein an amplified RF signal 906 and to radiate a radio wave through the antenna.
A distortion adding section 910 is to input therein an IQ signal 904 controlled in amplitude and phase and to output an IQ signal 911 added with a distortion.
A frequency converting section 912 is to input therein an IQ signal 911 added with a distortion and to output an RF signal 906.
Furthermore, FIG. 14 shows one configuration example of an amplitude-phase control section 903 of a conventional array antenna apparatus.
The I signal 1001 and the Q signal 1002, generated in the signal generating section, are respectively multiplied by weighting functions X and Y for amplitude weighting and phase rotation. These are converted into an I signal 1005 amplitude-weighted and phase-rotated and a Q signal 1006 amplitude-weighted and phase-rotated. Meanwhile, the weighting functions X and Y used in this time are read out of the values of a correction value table 1004 determined by the beam-direction control signal 1003. This correction value table 1004 is known to be determined by previously measuring a distortion of a singular power amplifier to be used and compute a proper correction value by storing a previously computed correction value or feeding back an output signal of the power amplifier. Incidentally, φ in the correction value data 1004 shows a phase angle (this is true for the subsequent figures).
Meanwhile, FIG. 15 shows an configuration example of an amplitude-phase distortion adding section 910 of a conventional array antenna apparatus.
The I signal 1201 and the Q signal 1202, amplitude-weighted and phase-rotated in the amplitude-phase control section 903, are respectively multiplied by weighting coefficients X and Y in order to add a distortion in an amplitude direction and phase direction. Then, these are converted into an I signal 1204 added with an amplitude distortion and phase distortion and a Q signal 1205 added with an amplitude distortion and phase distortion. Meanwhile, the coefficients X and Y used to add a distortion in the amplitude and phase directions use a value of correction value table 1203 read out in accordance with an instantaneous power of the input I signal 1201 and Q signal 1202. The correction value table 1203 is known to be determined by previously measuring a distortion of a power amplifier to be used and compute a proper correction value by storing a previously computed correction value or feeding back an output signal of the power amplifier. Incidentally, I2+Q2 in the correction value data 1203 shows an instantaneous power (this is true for the subsequent figures).
Meanwhile, conventionally, there is something like a description in JP-A-2002-190712 as an array antenna apparatus of this kind. FIG. 16 shows a configuration of the conventional array antenna apparatus described in the publication.
In FIG. 16, a transmission base-band signal 1501 is inputted to the frequency characteristic equalizing section 1502, to compensate for a frequency distortion occurred in each antenna array. The frequency characteristic equalizing section 1502 can be configured by a transversal filter. The frequency characteristic equalizing section 1502 has an output whose amplitude and phase is controlled for forming a beam by an amplitude-phase control section 1503. The amplitude-phase control section 1503 has an output to be input to a distortion compensating characteristic adding section 1504. In the distortion compensating characteristic adding section 1504, the input signal is added by a reverse characteristic to a nonlinear distortion occurred in a power amplifier 1506, depending upon an amplitude value of the input signal. The output of the distortion compensating characteristic adding section 1504, in a frequency converting section 1505, is converted into an RF band signal, and the output of the frequency converting section 1505 is amplified up to a required level by a power amplifier 1506. The power amplifier 1506 outputs a linear signal compensated for distortion whereby the signals sent at antennas 1507 are spatially combined together into a beam having a desired directivity. Meanwhile, a compensating-operation control section 1508 controls each distortion compensating characteristic adding section 1504 depending upon the information in a transmission power control signal 1509, thereby obtaining a desired transmission power.
However, the array antenna apparatus having distortion compensator circuits for the power amplifiers on part of antenna arrays has a problem that beam control accuracy deteriorates under the influence of a distortion caused by the power amplifier on the array not having a distortion adding section. Also, in the case of having a multiplicity of distortion compensator circuits, there is a problem that digital circuit increases in configuration to require a high consumption power.
Particularly, as compared to a QPSK modulation signal, when sending an OFDM or CDMA modulation signal having high peak vs. mean power ratio (PMPR), a difference in nonlinear distortion at between the power amplifiers in plurality is increased between upon transmitting a great power level signal and upon transmitting a small power level signal, resulting in deteriorated beam control accuracy.
The present invention has been made in order to solve the conventional problem, and it is an object thereof to provide an array antenna apparatus that nonlinear distortion is compensated, circuit configuration on the transmission system is size-reduced and consumption power efficiency is improved.